System and method for rapid heating of fluid

ABSTRACT

Apparatus for rapidly heating fluid includes a fluid circuit having electrodes between which the fluids flows. A voltage is applied between a pair of electrodes whereby current is caused to flow through the fluid. The inlet and outlet fluid temperatures are measured and the current controlled by varying the applied voltage to produce a desired temperature rise in the fluid in accordance with measured fluid flow rate.

FIELD OF THE INVENTION

The present invention relates to an apparatus, a system and method forthe rapid heating of fluid and more particularly, to an apparatus,system and method for rapidly heating fluid using electrical energy.

BACKGROUND OF THE INVENTION

Hot water systems of one form or another are installed in the vastmajority of residential and business premises in developed countries. Insome countries, the most common energy source for the heating of wateris electricity.

Of course, as it is generally known, the generation of electricity bythe burning of fossil fuels significantly contributes to pollution andglobal warming. For example, in 1996, the largest electricity consumingsector in the United States were residential households, which wereresponsible for 20% of all carbon emissions produced. Of the totalcarbon emissions from this electricity-consuming sector, 63% weredirectly attributable to the burning of fossil fuels used to generateelectricity for that sector.

In developed nations, electricity is now considered a practicalnecessity for residential premises and with electricity consumption perhousehold growing at approximately 1.5% per annum since 1990 theprojected increase in electricity consumption for the residential sectorhas become a central issue in the debate regarding carbon stabilisationand meeting the goals of the Kyoto Protocol.

From 1982 to 1996 the number of households in the United Statesincreased at a rate of 1.4% per annum and residential electricityconsumption increased at a rate of 2.6% per annum for the same period.Accordingly, the number of households in the United States is projectedto increase by 1.1% per annum through to the year 2010 and residentialelectricity consumption is expected to increase at a rate of 1.6% perannum for the same period.

It was estimated in 1995 that approximately 40 million householdsworldwide used electric water heating systems. The most common form ofelectric hot water heating system involves a storage tank in which wateris heated slowly over time to a predetermined temperature. The water inthe storage tank is maintained at the predetermined temperature as wateris drawn from the storage tank and replenished with cold inlet water.Generally, storage tanks include a submerged electricalresistance-heating element connected to the mains electricity supplywhose operation is controlled by a thermostat or temperature-monitoringdevice.

Electric hot water storage systems are generally considered to be energyinefficient as they operate on the principle of storing and heatingwater to a predetermined temperature greater than the temperaturerequired for usage, even though the consumer may not require hot wateruntil some future time. As thermal energy is lost from the hot water inthe storage tank, further consumption of electrical energy may berequired to reheat that water to the predetermined temperature.Ultimately, a consumer may not require hot water for some considerableperiod of time. However, during that time, some electric hot waterstorage systems continue to consume energy to heat the water inpreparation for a consumer requiring hot water at any time.

Of course, rapid heating of water such that the water temperaturereaches a predetermined level within a short period of time enables asystem to avoid the inefficiencies that necessarily occur as a result ofstoring hot water. Rapid heating or “instant” hot water systems arecurrently available where both gas, such as natural gas or LPG(Liquefied Petroleum Gas) and electricity are used as the energy source.In the case of natural gas and LPG, these are fuel sources that areparticularly well suited to the rapid heating of fluid as the ignitionof these fuels can impart sufficient thermal energy transfer to fluidand raise the temperature of that fluid to a satisfactory level within arelatively short time under controlled conditions.

However, whilst it is possible to use natural gas fuel sources for therapid heating of water, these sources are not always readily available.In contrast, an electricity supply is readily available to mosthouseholds in developed nations.

There have been previous ineffective attempts to produce an electrical“instant” hot water system. These include the hot wire and theelectromagnetic induction systems. The hot wire “instant” hot watersystem has been developed wherein a wire is located in a thermally andelectrically non-conductive tube of a relatively small diameter. Inoperation, water passes through the tube in contact with or in veryclose proximity to the wire, which is energised to thereby transferthermal energy to the water in the tube. Control is generally affectedby monitoring the output temperature of water from the tube andcomparing it with a predetermined temperature setting. Dependent uponthe monitored output temperature of the water, a voltage is applied tothe wire until the temperature of the water reaches the desiredpredetermined temperature setting. Whilst this type of system avoids theenergy inefficiencies involved with the storage of hot water, itunfortunately suffers a number of other disadvantages. In particular, itis necessary to heat the wire to temperatures much greater than that ofthe surrounding water. This has the disadvantageous effect of causingthe formation of crystals of dissolved salts normally present in varyingconcentrations in water such as calcium carbonate and calcium sulphate.Hot areas of the wire in direct contact with the water provide anexcellent environment for the formation of these types of crystals whichresults in the wire becoming “caked” and thus reducing the efficiency ofthermal transfer from the wire to the surrounding water. As the tube isgenerally relatively small in diameter, the formation of crystals canalso reduce the flow of water through the tube. In addition, hot wiretype systems require relatively high water pressures for effectiveoperation and thus these systems are not effective for use in regionsthat have relatively low water pressure or frequent drops in waterpressure that may occur during times of peak water usage.

The electromagnetic induction system functions like a transformer. Inthis case currents induced into a secondary winding of the transformercause the secondary winding to heat up. The heat generated here isdissipated by circulating water through a water jacket that surroundsthe secondary winding. The heated water is then passed out of the systemfor usage. Control is generally affected by monitoring the outputtemperature of water from the water jacket and comparing it with apredetermined temperature setting. Dependent upon the monitored outputtemperature of the water, voltage applied to the primary winding can bevaried, which varies the electric currents induces in the secondarywinding until the temperature of the water reaches the desiredpredetermined temperature setting. Whilst this type of system avoids theenergy inefficiencies involved with the storage of hot water, it alsosuffers a number of other disadvantages. In particular, it is necessaryto heat the secondary winding to temperatures greater than that of thesurrounding water. This has the same effect of causing the formation ofcrystals of dissolved salts as discussed above. As the gap between thesecondary winding and the surrounding water jacket is generallyrelatively narrow, the formation of crystals can also reduce the flow ofwater through the jacket.

In addition, the magnetic fields developed and the high currents inducedin the secondary winding may result in unacceptable levels of electricalor RF noise. This electrical or RF noise can be difficult to suppress orshield, and affects other electromagnetic susceptible devices withinrange of the electromagnetic fields.

It is therefore desirable to provide apparatus for rapid heating offluid, particularly water, using electrical energy and which obviates atleast some of the disadvantages of other systems.

It is also desirable to provide an improved method for rapidly heatingwater using electrical energy which minimises power consumption.

It is also desirable to provide an improved system for heating waterusing electrical energy which provides relatively rapid water heatingsuitable for domestic and/or commercial purposes.

It is also desirable to provide an improved apparatus and method forelectric fluid heating which facilitates control of the outputtemperature whilst minimising formation of crystals of dissolved salts.

It is also desirable to provide an improved fluid heating system whichuses mains power generally available in domestic and commercialbuildings.

It is also desirable to provide an improved heating apparatus which canbe manufactured in various capacities of fluid throughput.

It is also desirable to provide fluid heating apparatus which can bedesigned to operate with a variety of fluids or with water of varyinghardness.

It is also desirable to provide fluid heating apparatus which can beinstalled in close proximity to the hot water outlet, thereby reducingthe time delay of the arrival of hot water and thereby obviatingunnecessary wastage of water.

It will be understood that any discussion of devices, articles or thelike which has been included in this specification is solely for thepurpose of providing a context for the present invention. It is not tobe taken as an admission that any or all of these matters either formpart of the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is providedapparatus for heating fluid comprising passageway means defining a flowpath for the fluid to be heated, upstream fluid temperature measuringmeans to measure the temperature of fluid to be heated, a plurality ofsets of electrode means in or forming the flow path and between whichsaid fluid passes, said sets of electrode means including at least firstand second electrode sets along the fluid flow path, said firstelectrode set and said second electrode set both having at least onepair of electrodes between which an electric current is passed throughthe said fluid to heat the fluid during its passage along the flow path,first downstream temperature measuring means downstream of the secondelectrode set, fluid flow rate determining means, and electrical controlmeans to supply and control electrical power to the electrodes of eachset, said control means having processing means to relate current flowand applied voltage in response to measured upstream and downstreamtemperatures and fluid flow rate to determine desired power input to thefluid from each electrode set to achieve a desired fluid temperaturedownstream of the second electrode set.

Preferably, the passageway means comprises an annular space betweenspaced, substantially coaxial cylindrical members. The passageway meansmay define a plurality of parallel flow paths for the fluid.

In one embodiment, a second temperature measuring means measures thetemperature of the fluid between the first and second electrode sets,and the control means controls power to the first and second electrodesets in accordance with the measured temperatures and a desired fluidtemperature increase in the passage of the fluid between the respectiveelectrode sets.

In a preferred embodiment, the electrode means comprises at least threepairs of electrodes spaced along the flow path. The electrodes of eachpair are spaced across the flow path so that voltage applied between theelectrodes of each pair causes current to flow through the fluid acrossthe flow path as the fluid passes along the passageway means.

In one preferred embodiment, the electrode means comprises cylindrical,substantially coaxial electrodes forming or located in a section of thepassageway means. Preferably, the passageway means includes threesections, each passageway section having an inlet and an outlet, thesections being connected together in series such that the outlet of afirst section comprises the inlet of the second section, and the outletof the second section comprises the inlet of the third section, withelectrodes for each of the three sections.

With this arrangement, the outlets of the first and second sections havefluid temperature measuring means, and the control means controls thepower to the electrodes of each section in accordance with the measuredinlet and outlet temperatures of each section and a predetermineddesired temperature difference.

In a preferred embodiment, each passageway section is formed by spaced,substantially coaxial cylindrical electrodes defining an annular flowpath for the fluid.

In another embodiment, the passageway means includes more than threepassageway sections, each section having an inlet and an outlet, thesections being connected in series and the control means controllingpower to an electrode pair of each section in accordance with measuredinlet and outlet temperatures of each section and a predetermineddesired temperature difference for each section.

In preferred embodiments of the invention, control of the electricalpower being passed to the fluid is provided by a microcomputercontrolled management system. The microcomputer controlled managementsystem is preferably able to detect and accommodate changes in thespecific conductance of the fluid itself due to the change intemperature of the fluid within the system itself, as well as variancesin electrical conductivity of the incoming fluid. That is, in preferredembodiments of the present invention, the management system monitors andresponds to an electrical conductivity, or specific conductance gradientbetween the input and output of elements of the heating system. In aninstant fluid heating system in accordance with an embodiment of thepresent invention used for domestic water heating, fluctuations inincoming water electrical conductivity can also be caused by factorssuch as varying water temperatures and varying concentrations ofdissolved chemicals and salts, and such variations should be managed asa matter of course. However, preferred embodiments of the presentinvention will also manage and respond to changes in the electricalconductivity of the fluid as it is heated within the system itself, thatis, the effective management of the specific conductance gradient.

According to another aspect of the invention there is provided a methodfor heating fluid comprising the steps of:

-   -   passing fluid along a fluid path;    -   providing at least two sets of electrodes spaced along the fluid        path;    -   applying a variable electrical voltage between the electrodes of        each set to thereby pass electrical currents through the fluid        between electrodes of each set;    -   monitoring fluid path inlet fluid temperature;    -   monitoring fluid path outlet fluid temperature;    -   monitoring the currents passing through the fluid between        electrodes of each electrode set in response to application of        the variable electrical voltage; and    -   controlling the variable electrical voltage between electrodes        of each electrode set in response to the specific conductance of        the fluid as determined by reference to the monitored fluid        temperatures and current flows for a given fluid flow in each        section of the flow path such that an amount of electrical power        passed to the fluid corresponds to a predetermined temperature        increase of the fluid.

In preferred embodiments of the method of the present invention,additional further steps may be carried out comprising:

Compensating for a change in the electrical conductivity of the fluidcaused by varying temperatures and varying concentrations of dissolvedchemicals and salts, and through the heating of the fluid, by alteringthe variable electrical voltage to accommodate for changes in specificconductance when increasing the fluid temperature by the desired amount.

Such a step may be performed by controlling the electrical power appliedto the electrode sets to maintain the required constant fluidtemperature increase in that electrode segment. The variable electricalvoltage may then be adjusted to compensate for changes in specificconductance of the fluid within the segment of the flow path associatedwith each electrode pair, which will affect the current drawn by thefluid in that segment. The changes in specific conductance of the fluidpassing through the separate electrode segments can be managedseparately in this manner. Therefore the system is able to effectivelycontrol and manage the resulting specific conductance gradient acrossthe whole system.

Similarly, the system of the present invention preferably furthercomprises means to manage the changes in specific conductance of thefluid caused by heating of the fluid. Such means may comprise atemperature sensor for measuring the system output fluid temperature forcomparison to the input fluid temperature of each section in order todetermine whether a desired temperature increase of the fluid has beenachieved.

In preferred embodiments, a temperature sensor is placed upstream fromthe electrode segments to supply a signal representative of thetemperature of the fluid prior to its passage between the electrodesegments. With the temperature sensor placed upstream of the electrodesegments, a temperature difference may be determined between the inletfluid and a desired temperature of the outlet fluid. The desiredtemperature of the outlet fluid may be adjusted by a user via anadjustable control means.

The volume of fluid passing between any set of electrodes may beaccurately determined by measuring the dimensions of the passage withinwhich the fluid is exposed to the electrodes taken in conjunction withfluid flow.

Similarly, the time for which a given volume of fluid will receiveelectrical power from the electrodes may be determined by measuring theflow rate of fluid through the passage. The temperature increase of thefluid is proportional to the amount of electrical power applied to thefluid. The amount of electrical power required to raise the temperatureof the fluid a known amount, is proportional to the mass (volume) of thefluid being heated and the fluid flow rate through the passage. Themeasurement of electrical current flowing through the fluid can be usedas a measure of the electrical conductivity, or the specific conductanceof that fluid and hence allows determination of the required change inapplied voltage required to keep the applied electrical power constant.The electrical conductivity, and hence the specific conductance of thefluid being heated will change with rising temperature, thus causing aspecific conductance gradient along the path of fluid flow.

The energy required to increase the temperature of a body of fluid maybe determined by combining two relationships:

Relationship (1)Energy=Specific Heat Capacity×Density×Volume×Temp-Changeor

-   The energy per unit of time required to increase the temperature of    a body of fluid may be determined by the relationship:    ${{Power}(P)} = \frac{\begin{matrix}    {{Specific}\quad{Heat}\quad{{Capacity}({SHC})} \times} \\    {{Density} \times {{Vol}(V)} \times {Temp}\text{-}{{Change}({Dt})}}    \end{matrix}}{{Time}(T)}$    .

For analysis purposes, the specific heat capacity of water may beconsidered as a constant between the temperatures of 0 degC. and 100degC. The density of water being equal to 1, may also be consideredconstant. Therefore, the amount of energy required to change thetemperature of a unit mass of water, 1 degC. in 1 second is consideredas a constant and can be labelled “k”. Volume/Time is the equivalent offlow rate (Fr). Thus:

-   The energy per unit of time required to increase the temperature of    a body of fluid may be determined by the relationship:    ${{Power}(P)} = \frac{k \times {Flow}\quad{{rate}({Fr})} \times {Temp}\text{-}{{Change}({Dt})}}{{Time}(T)}$    .

Thus if the required temperature change is known, the flow rate can bedetermined and the power required can be calculated.

In preferred embodiments of the present invention, the electrodes aresegmented and input and output temperatures are measured. Measurement ofthe temperature allows the computing means of the microcomputercontrolled management system to determine the voltage required to beapplied to the electrodes in an electrode segment in order to supply anecessary amount of electrical power to the fluid in order to providethe necessary management of changes in the specific conductance of thefluid so as to increase the temperature of the fluid by a desiredamount.

Typically, when a user requires heated water, a hot water tap isoperated thus causing water to flow. This flow of water may be detectedby a flow meter and cause the initiation of a heating sequence. Thetemperature of incoming water may be measured and compared with a presetdesired temperature for water emitting from the system. From these twovalues, the required change in water temperature from inlet to outletmay be determined.

Of course, the temperature of the inlet water to the electrode segmentsmay be repeatedly measured over time and as the value for the measuredinlet water temperature changes, the calculated value for the requiredtemperature change from inlet to outlet of the electrode segments can beadjusted accordingly. Similarly, with changing temperature, mineralcontent and the like, changes in electrical conductivity and thereforespecific conductance of the fluid may occur over time. Accordingly, thecurrent passing through the fluid will change causing the resultingpower applied to the water to change. Repeatedly measuring thetemperature outputs of the electrode segments over time and comparingthese with the required output temperature values will enable repeatedcalculations to correct the voltage applied to the electrode segments.

In one preferred embodiment, a computing means provided by themicrocomputer controlled managemerit system is used to determine theelectrical power that should be applied to the fluid passing between theelectrodes, by determining the value of electrical power that willeffect the desired temperature change between the electrode segmentinlet and outlet, measuring the effect of changes to the specificconductance of the water and the thereby calculate the voltage thatneeds to be applied for a given flow rate.

Control of Electrical Power

In preferred embodiments of the present invention, the electricalcurrent flowing between the electrodes within each electrode segment,and hence through the fluid, is measured. The electrode segment inputand output temperatures are also measured. Measurement of the electricalcurrent and temperature allows the computing means of the microcomputercontrolled management system to determine the power required to beapplied to the fluid in an electrode segment to increase the temperatureof the fluid by a desired amount.

The current passing through the fluid will change. The current passingthrough the fluid is preferably measured repeatedly over time to enablerepeated calculations, which allows the electrical power applied to theelectrode segments to remain at the appropriate value.

In one embodiment, the computing means provided by the microcomputercontrolled management system determines the electrical power that shouldbe applied to the fluid passing between the electrodes and the therebycalculate the average voltage that needs to be applied to keep thetemperature change substantially constant.

Relationship (2) below, facilitates the calculation of the electricalpower to be applied as accurately as possible, almost instantaneously.This eliminates the need for unnecessary water usage otherwise requiredto initially pass through the system before facilitating the delivery ofwater at the required temperature. This provides the potential forsaving water.

In the preferred embodiments, having determined the electrical powerthat should be supplied to the fluid passing between the electrodes, thecomputing means may then calculate the voltage that should be applied toeach Electrode Segment (ES) as follows if the Power required for theelectrode segment can be calculated, and the current drawn by theelectrode segment (n) can be measured:

Relationship (2)Voltage ESn(Vappn)=Power ESn (Preqn)/Current ESn(Isn)Vappn=Preqn/Isn

As part of the initial heating sequence, the applied voltage may be setto a relatively low value in order to determine the initial specificconductance of the fluid passing between the electrodes. The applicationof voltage to the electrodes will cause current to be drawn through thefluid passing therebetween thus enabling determination of the specificconductance of the fluid, as it is directly proportional to the currentdrawn therethrough. Accordingly, having determined the electrical powerthat should be supplied to the fluid flowing between the electrodes inthe electrode segments, it is possible to determine the required voltagethat should be applied to those electrodes in order to increase thetemperature of the fluid flowing between the electrodes in the electrodesegments by the required amount. The instantaneous current being drawnby the fluid is preferably continually monitored for change along thelength of the heating path. Any change in instantaneous current drawn atany position along the heating path is indicative of the change inelectrical conductivity or specific conductance of the fluid. Thevarying values of specific conductance apparent in the fluid passingbetween the electrodes in the electrode segments, effectively definesthe specific conductivity gradient along the heating path.

Preferably, various parameters are continuously monitored andcalculations continuously performed to determine the electrical powerthat should be supplied to the fluid and the voltage that should beapplied to the electrodes in order to raise the temperature of the fluidto a preset desired temperature in a given period.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a schematic block diagram of a rapid heating system accordingto one embodiment of the present invention;

FIG. 2 is a legend of some of the symbols used in FIG. 1; and

FIG. 3 is an illustration of one form of electrode segment assembly inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF DESCRIBED EMBODIMENTS OF THE INVENTION

Referring to the drawings, FIG. 1 shows a schematic block diagram of aheating system of one embodiment in which water is caused to flowthrough a pipe or tube 12. The tube 12 is preferably made from amaterial that is electrically non-conductive, such as synthetic plasticmaterial. However, the tube 12 is likely to be connected to metallicwater pipe, such as copper tubing, that is electrically conductive.Accordingly, earth straps 14 are included at each end of the tube 12 forelectrically earthing any metal tubing connected to the tube 12. Theearth straps 14 would ideally be connected to an electrical earth of theelectrical installation in which the heating system of the embodimentwas installed. As the earth straps may draw current from an electrodethrough water passing through the tube 12, activation of an earthleakage circuit breaker or residual current device (RCD) may beeffected. In a particularly preferred form of this embodiment, thesystem includes earth leakage circuit protective devices.

The tube 12, which defines the fluid flow path, is provided with threesets of electrodes 16, 17 and 18. The electrode material may be anysuitable metal or a non-metallic conductive material such as conductiveplastics material, carbon impregnated material or the like. It isimportant that the electrodes are selected of a material to minimisechemical reaction and/or electrolysis.

One of each electrode pair 16 b, 17 b and 18 b is connected to a commonswitched return path 19 via separate voltage supply power controldevices 21 a, 22 a and 23 a while the other of each electrode pair 16 a,17 a and 18 a are connected to the incoming three phase voltage supply21, 22 and 23. The separate voltage supply power control devices 21 a,22 a and 23 a switch the common return in accordance with the powermanagement control. The total electrical current supplied to theelectrodes is measured by the current measurement device 26, while thecurrent supplied to each individual pair of electrodes 16, 17 and 18 ismeasured by current measuring devices 27, 28 and 29, respectively. Asignal representing the total current is supplied as an input signalalong line 31 via plug J6 and signal input interface 33 to power supplycontroller 41. The power supply controller 41 also receives signalsdirectly from a flow measurement device 34 located in the tube 12 and atemperature setting device 37 by which a user can set a desired outputfluid temperature, and additional signals via the plug J6 and signalinput interface from an input temperature measurement device 35 tomeasure the temperature of input fluid to the tube 12, an outputtemperature measurement device 36 measuring the temperature of fluidexiting the tube 12, a first intermediate temperature measurement device38 to measure fluid temperature between the electrodes 16 and 17, and asecond intermediate temperature measurement device 39 to measure thetemperature of fluid between the set of electrodes 17 and 18.

The power controller 41 receives the various monitored inputs andperforms necessary calculations with regard to desired electrode pairvoltages to provide a calculated power to be supplied to the fluidflowing through the tube 12. The power controller 41 controls the pulsedsupply of voltage from each of the three separate phases connected toeach of the electrode pairs 16, 17 and 18. Each pulsed voltage supply isseparately controlled by the separate control signals from the powercontroller 41 to each voltage supply power control device 21 a, 22 a and23 a.

It will therefore be seen that, based upon the various parameters forwhich the power controller 41 receives representative input signals, acomputing means under the control of a software program within the powercontroller 41 calculates the control signals required by the respectivevoltage supply power control device 21 a, 22 a and 23 a in order tosupply a the required electrical power to impart the requiredtemperature change in the water flowing through the tube 12 so thatheated water is emitted from the tube 12 at the desired temperature setby the set temperature device 37.

When a user sets the desired output water temperature using the settemperature device 37, the set value is captured by the power controller41 and stored in a system memory until it is changed or reset.Preferably, a predetermined default value is retained in the memory, andthe set temperature device 37 may provide a visual indication of thetemperature set. The power controller 41 may have a preset maximum forthe set temperature device 37 which represents the maximum temperaturevalue above which water may not be heated. Thus, the value of the settemperature device 37 cannot be greater than the maximum set value. Thesystem may be designed so that, if for any reason, the temperaturesensed by the output temperature device 36 was greater than the setmaximum temperature, the system would be immediately shut down anddeactivated.

The system is actuated when water flow is detected by the flowmeasurement device 34 via plug J7. This causes initiation of a heatingsequence. The temperature of incoming water is measured by the inputtemperature device 35 and this value is captured by the input interface33 and recorded in the system memory. With the set temperature device 37having a set or default temperature value, the required change in watertemperature is easily determined, being the difference in the settemperature and measured input temperature. Of course, the temperatureof the inlet water is repeatedly measured and if the value changes, thecalculated temperature difference also varies.

The computing means is then able to determine the electrical power thatneeds to be applied to the water flowing through the tube 12 in order toincrease its temperature from the measured input temperature to the settemperature. Having calculated the electrical power that needs to beapplied to the flowing water, the computing means is then able tocalculate the voltage that needs to be applied between the pairs ofelectrodes 16, 17 and 18 to thereby cause the required current to flowthrough the water.

As part of an initial heating sequence, the applied voltage is set to apredetermined low value in order to calculate the water conductivity, orspecific heat capacity. The application of this voltage to the waterwill cause current to be drawn, and the current measuring device 26 willmeasure the drawn current and provide a signal to the input interface33. The value of the total current is also measured periodically.

The control system then performs a series of checks to ensure that:

-   -   (a) the water temperature at the outlet does not exceed the        maximum allowable temperature;    -   (b) leakage of current to earth has not exceeded a predetermined        set value; and    -   (c) system current does not exceed a preset current limit of the        system.

These checks are repeatedly performed while the unit is operational andif any of the checks reveals a breach of the controlling limits, thesystem is immediately deactivated.

When the initial system check is satisfactorily completed, a calculationis performed to determine the required voltage that must be applied tothe water flowing through the tube 12 in order to change its temperatureby the desired amount. The calculated voltage is then applied to thepairs of electrodes 16, 17 and 18 so as to quickly increase the watertemperature as it flows through the tube 12.

As the water flowing through the tube 12 increases in temperature fromthe inlet end of the tube, the conductivity changes in response toincreased temperature. The first and second intermediate temperaturemeasuring devices 38 and 39 and the output temperature measuring device36 measure the incremental temperature increases in the three segmentsof the tube 12 containing the electrodes 16, 17 and 18. The voltage thenapplied across the respective pairs of electrodes 16, 17 and 18 can thenbe varied to take account of the changes in water conductivity to ensurethat an even temperature rise occurs along the length of the tube 12, tomaintain a substantially constant power input by each of the sets ofelectrodes and to ensure greatest efficiency and stability in waterheating between the input temperature measurement and the outputtemperature measurement. The power supplied to the flowing water ischanged by increasing or decreasing the number of control pulsessupplied to power control devices 21 a, 22 a and 23 a. The serves toincrease or decrease the power supplied by individual electrode pairs16, 17 and 18 to the water.

It will be seen that the system repeatedly monitors the water forchanges in conductivity by continuously interrogating the system currentmeasurement device 26 and the individual current measuring devices 27,28 and 29, and the temperature measurement devices 35, 36, 38 and 39.Any changes in the values for the water temperatures as detected alongthe length of the tube 12 or changes in the detected currents cause thecomputing means to calculate revised average voltage values to beapplied across the electrode pairs. Constant closed loop monitoring ofchanges to the system current, individual electrode currents, electrodesegment water temperature causes recalculating of the voltage to beapplied to the individual electrode segments to enable the system tosupply the appropriate stable power td the water flowing through theheating system.

FIG. 2 illustrates another representation of the embodiment of theinvention in which the water is caused to flow through three cells 51,52 and 53 which are connected together by connecting pipes 54 and 55. Awater inlet pipe 56 enables water to be supplied to the cells 51, 52 and53 and a water outlet pipe 57 conveys water from the system.

The system is controlled by a control unit 58 mounted on one side of thesystem, and the control unit 58 is connected to a source of mains powerat 59. With the system of this embodiment, each individual cell 51, 52and 53 houses one set of electrodes between which the water flowingthrough the system passes. Each cell, therefore, receives water at aninlet temperature determined by the relative positioning of the cell.

The control system of this embodiment operates in a similar manner tothat described above. The inlet and outlet temperatures of the waterflowing through each cell are continuously measured and the voltagesapplied between the electrodes of each cell are controlled so that therequired power is supplied in each cell to evenly raise the temperatureof the water flowing from the inlet pipe 56 to the outlet pipe 57.

Referring to FIG. 3, the individual cells of FIG. 2 may each comprise apair of coaxial electrodes 61 and 62, radially spaced to define anannular fluid flow path 63. The outer electrode 61 is formed of a tubeof electrically conductive material and a fluid inlet 64 enables fluidto pass into the annular flow path 63 between the electrodes 61 and 62.

The inner electrodes 62 may also comprise a tube of electricallyconductive material coaxially mounted within the outer electrode 61. Thecoaxial mounting may comprise a plug 66 fastened in one end of the innerelectrode 62 a spacer 67 engaged over the end of the inner electrode 62and within the end of the outer electrode 61, and an end cap 68 fastenedto the plug 66 by a bolt 69. The spacer 67 is an interference or closefit within the annular space between the inner and outer electrodes 61and 62 to thereby seal that space and confine fluid to the flow pathwithout fluid leakage.

A cell formed in this manner enables a voltage to be applied between theinner and outer electrodes 61 and 62 such that current is able to flowbetween the surfaces of the electrodes and through the fluid flowingthrough the annular flow path 63. Connections for electrical supply tothe inner electrode 62 may be through the end plug 66 and end cap 68.

The outer electrode 61 may be encased in an insulating material toprovide a safe electrical environment for the water heating system. Suchan insulating material may comprise a plastic tube or the like whichclosely engages the external surface of the outer electrode 61.

It will be appreciated that any number of sets of electrodes may be usedin the performance of the present invention. Thus, while the embodimentsdescribed show three electrode pairs, the number of electrodes may beincreased or decreased in accordance with individual requirements forfluid heating. If the number of electrodes is increased to, for example,six pairs, each individual pair may be individually controlled withregards to electrode voltage in the same way as is described in relationto the embodiments herein.

1. An apparatus for heating fluid comprising passageway means defining aflow path for the fluid to be heated, upstream fluid temperaturemeasuring means to measure the temperature of fluid to be heated, aplurality of sets of electrode means in or forming the flow path andbetween which said fluid passes, said sets of electrode means includingat least first and second electrode sets along the fluid flow path, saidfirst electrode set and said second electrode set both having at leastone pair of electrodes between which an electric current is passedthrough the said fluid to heat the fluid during its passage along theflow path, first downstream temperature measuring means downstream ofthe second electrode set, fluid flow rate measuring means, andelectrical control means to supply and control electrical power to theelectrodes of each set, said control means having processing means torelate current flow and applied voltage in response to measured upstreamand downstream temperatures and fluid flow rate to determine desiredpower input to the fluid from each electrode set to achieve a desiredfluid temperature downstream of the second electrode set.
 2. Anapparatus according to claim 1 wherein said passageway means comprisesan annular space between spaced, substantially coaxial cylindricalmembers.
 3. An apparatus according to claim 1 wherein a secondtemperature measuring means measures the temperature of the fluidbetween the first and second electrode sets, and the control meanscontrols power to the first and second electrode sets in accordance withthe measured temperatures and a desired fluid temperature increase inthe passage of the fluid between the respective electrode sets.
 4. Anapparatus according to claim 1 wherein said plurality of sets ofelectrode means includes a third electrode set positioned downstream ofsaid second electrode set, and a third downstream temperature measuringmeans measures the fluid temperature downstream of the third electrodeset.
 5. An apparatus according to claim 1 wherein the electrode meanscomprises cylindrical, substantially coaxial electrodes definingseparate sections of the passageway means along the flow path(s).
 6. Anapparatus according to claim 1 wherein said passageway means includesthree sections, each passageway section having an inlet and an outlet,the sections being connected together in series such that the outlet ofa first section communicates with the inlet of the second section, andthe outlet of the second section communicates with the inlet of thethird section, with a set of electrodes for each section.
 7. Anapparatus according to claim 6 wherein the outlets of each of the firstsecond and third sections have fluid temperature measuring means, andsaid control means controls the power to the electrodes of each sectionin accordance with the sensed inlet and outlet temperatures of eachsection and a predetermined desired temperature difference.
 8. Anapparatus according to claim 6 wherein each passageway section is formedby spaced substantially coaxial cylindrical electrodes defining anannular flow path for the fluid.
 9. An apparatus according to claim 1wherein said passageway means includes more than three passagewaysections, each section having an inlet and outlet, the sections beingconnected in series and the control means controlling power to anelectrode pair of each section in accordance with measured inlet andoutlet temperatures of each section, and a predetermined desiredtemperature difference.
 10. An apparatus according to claim 7 whereinthe predetermined desired temperature difference is determined inrelation to applied voltage between the respective electrodes andcurrent drawn, inlet and outlet temperatures of the sections, fluid flowand upstream and downstream measures temperatures.
 11. An apparatusaccording to claim 1 wherein the control means supplies a varyingvoltage to the electrode pairs at a pulse frequency which issub-multiple of mains supply voltage frequency, and control of the powersupplied to the electrodes includes varying the number of pulses perunit time.
 12. An apparatus according to claim 1 wherein said passagewaymeans defines a plurality of parallel flow paths for said fluid, eachflow path having a plurality of sets of electrode means in or formingthe flow path.
 13. A method for heating fluid comprising the steps of:passing fluid along a fluid path; providing at least two sets ofelectrodes spaced along the fluid path; applying a variable electricalvoltage between the electrodes of each set to thereby pass electricalcurrents through the fluid between electrodes of each set; monitoringfluid path inlet fluid temperature; monitoring fluid path outlet fluidtemperature; monitoring the currents passing through the fluid betweenelectrodes of each electrode set in response to application of thevariable electrical voltage; and controlling the variable electricalvoltage between electrodes of each electrode set in response to thespecific conductance of the fluid as determined by reference to themonitored fluid temperatures and current flows for a given fluid flow ineach section of the flow path such that an amount of electrical powerpassed to the fluid corresponds to a predetermined temperature increaseof the fluid.
 14. A method for heating fluid according to claim 13including the step of monitoring the temperature of the fluid betweenthe electrode sets.
 15. A method for heating fluid according to claim 13including the step of controlling the electrical power passed to thefluid by a microcomputer controlled management system.
 16. A method forheating fluid according to claim 13 including the step of managing andresponding to changes in the electrical conductivity of the fluid as itis heated within the system in conjunction with measured fluid flow,fluid inlet temperature, and desired rate of temperature rise.
 17. Amethod for heating fluid according to claim 13 including the step ofcompensating for a change in the electrical conductivity of the fluidcaused by varying temperatures and varying concentrations of dissolvedchemicals and salts, and through the heating of the fluid, by alteringthe average electrical voltage to accommodate for changes in specificconductance when increasing the fluid temperature by the desired amount.18. A method for heating fluid according to claim 13 including the stepsof providing at least three sets of electrodes in the fluid flow,applying an electrical voltage between electrodes of each pair inaccordance with monitored temperatures of fluid at locations upstreamand downstream of the electrode pairs.
 19. A method for heating fluidaccording to claim 18 including the steps of monitoring the temperatureof the fluid in the flow path on either side of each pair of electrodes,separately controlling the electrical power applied to the electrodepairs of each set of electrodes to maintain a required constant fluidtemperature increase in that segment of fluid flow adjacent therespective electrode pairs.
 20. A fluid heating system comprising atleast one flow path for the fluid to be heated and having a fluid inlet,inlet fluid temperature measuring means, at least two pairs ofelectrodes in or defining the fluid path, the electrode pairs beingspaced along the flow path, downstream fluid temperature measuring meansdownstream of each electrode pair, fluid flow rate measuring means,electrical control means to supply and control electrical power to theelectrodes of each pair, said control means having processing means torelate current flow, applied voltage, inlet fluid temperature,respective downstream fluid temperatures, and fluid flow rate todetermine desired power input to the fluid by each electrode pair toachieve a desired outlet fluid temperature in a predetermined time. 21.A fluid heating system according to claim 20 wherein said flow pathcomprises an annular space between spaced, substantially coaxialcylindrical members.
 22. A fluid heating system according to claim 20wherein said cylindrical members constitute said electrodes.
 23. A fluidheating system according to claim 20 having a plurality of parallel flowpaths for said fluid, each flow path having a plurality of sets ofelectrode means in or forming the flow path.
 24. A fluid heating systemaccording to claim 20 wherein said flow path includes three sections,each section having an inlet and an outlet, the sections being connectedtogether in series such that the outlet of a first section communicateswith the inlet of the second section, and the outlet of the secondsection communicates with the inlet of the third section, withelectrodes for each section.
 25. A fluid heating system according toclaim 20 wherein fluid temperature measuring devices are locatedadjacent each set of electrodes, and said control means controls thepower to the electrodes of each section in accordance with the sensedinlet and outlet temperatures of each section and a predetermineddesired temperature difference in each section.
 26. A fluid heatingsystem according to claim 20 wherein the control means supplies avoltage to the electrode pairs at a pulse frequency which issub-multiple of mains supply voltage frequency, control of the powersupplied to each pair of electrodes including control by varying thenumber of pulses.